The present application relates to phosphors. In particular, the application relates to nitride phosphors that can convert blue photons from a light emitting diode into frequencies within the red portion of the visible spectrum. The production of such red tones helps tailor the color produced by an LED lamp, and in particular red photons help produce warmer white light from LED lamps.
Lighting is done in a number of different fashions. Incandescent lighting uses the emission from a tungsten filament to produce the characteristic light. Fluorescent lighting uses an ultraviolet source (mercury emission) to strike a photoluminescent material that produces the white light emission from a fluorescent lamp. Halogen lighting also uses a tungsten filament but includes a trace of halogen gas (typically iodine or bromine) that helps regenerate the tungsten filament during operation which in turn increases the lifetime of the lamp. Solid state lamps emit photons when a current is directed across a p-n junction (or its equivalent).
Each of the relevant forms of lighting have corresponding advantages and disadvantages. Incandescent lighting typically produces warmer colors and bulbs and fixtures are widely available at low cost. From an energy efficiency standpoint, however, incandescent bulbs tend to produce most of their energy as heat rather than light. Indeed, future regulatory schemes may minimize the availability of the most common incandescent bulbs.
Fluorescent lighting tends to be more energy-efficient that incandescent lighting, but requires starter circuits and related hardware. As a result, cost-effective fluorescent lighting that can be used in incandescent fixtures has only recently been developed. Fluorescent bulbs also typically contain mercury, even though in minimal amounts. Solid state lighting has the advantage of long lifetime, higher energy efficiency and potentially lower cost, but has historically suffered from low brightness and (as indicated elsewhere) the unavailability of the relevant colors that can produce white light.
More recently, solid-state lighting has become commercially available based on the increased availability at competitive prices of lamps based on light emitting diodes that can produce white light. Although solid-state devices (light emitting diodes) have been used for indicator purposes for several decades, two factors limited or precluded the use of light emitting diodes as the basis for illumination: the lack of diodes that could produce the frequencies required to produce white light; and, once such diodes became available, their generally low brightness.
Advances in the art have reduced these (and other) barriers to solid state lighting. First, blue light emitting diodes have been available at competitive prices in commercial quantities for over a decade. The blue light emitting diode is a necessary component of white light because (as explained below) the blue photons are required either to contribute to a three color lamp or to excite an appropriate phosphor.
As a second advance, the brightness of available LED lamps continues to increase.
Because white light is a combination of many frequencies within the visible spectrum, it can be produced from blue, green and red primary sources. Thus, a lamp that emits white light can be produced from one or more red light emitting diodes, one or more green light emitting diodes and one or more blue light emitting diodes. This technique can be relatively complex, however, because of the number of diodes required.
In the recent growth of white light-emitting lamps using light emitting diode sources, the most common method has been to incorporate a light emitting diode that emits in the blue, violet or ultraviolet portions of the electromagnetic spectrum. Such a diode is then combined, usually in a package that includes a lens, with a phosphor that absorbs the blue (violet, UV) emission and produces a yellow emission in response. The combination of the blue light from the diode and the yellow light from the phosphor gives white light.
As well-understood in this art, a phosphor is a composition that generally absorbs a given frequency, or range of frequencies, of light and then emits different color photons, usually of a lower frequency and usually including a range of frequencies.
A typical phosphor is a solid composition that includes an (activator) ion in a host structure. Because light emitting diodes that will produce blue light are relatively new in commercial appearance (about a decade), the use of blue light emitting diodes combined with yellow phosphors to produce white light is also relatively recent.
Different white light sources, however, have slightly different appearances to the human eye. These are sometimes quantified using a well-recognized measurement referred to as color temperature. When stated descriptively, white light that is more bluish in tint is referred to as being cooler, while light that has more of a yellow or red component is generally referred to as being warmer. Depending upon the desired end use, cooler lamps are preferred in some circumstances while warmer whites are preferred in other circumstances. As one example, skin colors tend to look more natural under warmer lamps than under cooler ones.
In general, incandescent lighting is warmer than fluorescent lighting; although warmer fluorescent lamps are available. In any case, if LED lighting is to successfully replace incandescent and fluorescent lighting (for reasons in addition to its energy advantages), diode lamps that will emit with a red or yellow component to give a warmer appearance will be desired.
Because blue light emitting diodes are relatively recent, the need or commercial desire for phosphors that can convert a blue photon into a red emission in the context of an LED lamp is also relatively recent. One predominant source for such a phosphor is set forth in international application number WO2005052087 (and also published as US20070007494). This publication describes a nitride-based phosphor that is relatively recent in its commercial appearance. The phosphor composition is formed of materials that are highly reactive in water or air, and thus is relatively difficult to produce without sophisticated equipment.
In most typical LED applications, a phosphor must have color stability; i.e., its chemical composition must be consistent enough over the course of time so that the color of the light emitted by the lamp remains consistent. Stated differently, if the phosphor chemical composition breaks down relatively quickly, the color produced by the diode lamp will change quickly, and usually in an undesired manner.
The phosphor described in the '087 publication is also expensive, available only from limited sources, and because of manufacturing difficulty, is sometimes hard to obtain. For example, the silicon nitride that is typically one of the starting materials is relatively inert, even at high temperatures. Indeed, because of its high-temperature stability, silicon nitride is typically used as passivation for semiconductor components. Additionally, the alkaline earth metals that represent other starting materials react quickly (often too quickly) with oxygen and moisture.
Accordingly, a need exists for improved processes and resulting phosphor compositions that will produce a red emission, when stimulated by a blue photon that are stable in their composition and color output, and that are more easily manufactured than currently available phosphors having this characteristic.